If every budget has a theme, the raft of measures announced by Alistair Darling yesterday can be called the banking-crisis budget. Not just because the collapse of Lehman Brothers and all those other financial dominoes prompted major economic shocks which this government and others have to deal with. But also because the banking crisis has thrown open to question a decade of economic and political assumptions that Gordon Brown took for granted, yet which his successor at Number 11 cannot. What replaces New Labour's economic model now, with its reliance on booming finance, soaraway house prices and feel-good on the high street? What price light-touch regulation when giant banks are being part-nationalised? These questions will feature in any debate over economic policy and politics for years to come.

Coming after the seismic shocks of this autumn, Mr Darling's crisis statement yesterday - hardly a pre-budget report, being chunkier than many full-blown budgets - did not resolve those issues. It could not, so soon. But it showed a chancellor edging away from the assumptions and boasts of his big clunking predecessor - even if he was unsure what would replace them.

So Gordon Brown's famous fiscal rules were put in a drawer, not to be opened until the middle of next decade. That golden rule of borrowing only to invest in new buildings and the like now looks academic in the face of a worldwide downturn. As for the 40% cap on government borrowing, forget it. By 2013, Mr Darling admits, public debt could be over 57% of national income. The chancellor should have gone further and announced that he would overhaul the decade-old fiscal framework. After all, this recession has shown up the redundancy of a fixed limit of 40% and the need for a more sensible, flexible regime. Instead, Mr Darling announced he would return to the true Brownian path - after an eight-year hiatus. It was a compromise with old orthodoxy that the new chancellor did not need to make.

But that is what Mr Darling's performance yesterday was like: full of compromise, while at the same time showing a more freethinking side. That VAT giveaway, for instance, looked like classic Brown: an attempt to buy the support of Middle England by spraying a little cash everywhere. Yes, it will boost spending a bit. But when Sir Stuart Rose is offering 20% off at Marks and Spencer, an extra 2.5% discount does not make much odds. Yes, it will be easy to remove later, but it is a pointless policy, encouraging junk-consumerism of Chinese imports, rather than laying the ground for a more sustainable economic model. And that is what Britain needs, as the coming months will make clear, with banks laying off workers and tax income from financial firms, house sales and share-dealing fast drying up.

Contrast that sweetener with the radical edge Mr Darling showed elsewhere. His proposals to claw back the tax giveaway by giving a hefty chunk of the bill to the rich, with a new supertax and a squeeze on high-earners' tax allowances are welcome bits of redistribution. Two measures are bound to go less noticed today but they are about as radical as any government has got in tackling this financial crisis: it is proposing to underwrite £1bn of loans to small businesses and bank issuance of mortgage-backed securities. Unglamorous and technical they may be, but they are potentially far greater interventions in the market economy than even the oldest of Old Labour MPs would have thought possible just a few months ago.

Will Mr Darling's medicines work? Some may. The package of measures to help small businesses is excellent: targeted and thorough. The boost to consumers - the lion's share being that cut in VAT - looks more risky, although the targeting of payments to pensioners and the poor looks sensible, since these groups tend to spend more of their incomes. Even so, the economy is unlikely to have the short, sharp recession predicted by the government. The worst would be over by next summer, Mr Darling forecast, in a claim that has hostage-to-fortune written all over it. And the most crucial part of restoring the economy to any form of health lies not with more public spending, but with getting the banks lending again. Until then, predictions on growth or public finances are as much guesswork as analysis.

What was most remarkable about yesterday's PBR, though, was the possibilities it held out for future budgets. For over a decade, the two major parties have agreed that a tax cut here must be paid for by a tax rise there and that income tax rates should never go anywhere but down. That dogma is over; the argument now is over what comes next.